Undocumented workers screened passengers at Dallas-Fort Worth International Airport and seven of 20 screeners at Washington Dulles International Airport failed their annual tests, government investigators said Tuesday.

The Transportation Department's inspector general and the Federal Aviation Administration were looking at passenger screeners who staff checkpoints at 14 airports, part of an investigation into whether background checks required of security employees were done.

The probe began after federal prosecutors accused the company that handles security at the 14 airports, Argenbright Security Inc., of failing to make background checks at Philadelphia International Airport. Government investigators were sent to Philadelphia, and separate teams went to 13 other airports staffed by Argenbright employees.

Besides Dulles, Argenbright handles security at Boston and Newark. The four planes commandeered by terrorists in the attacks that killed more than 5,000 people flew from those three airports, although the two that originated in Boston were from airlines that do not use Argenbright.

Federal prosecutors charged last week that the company hired criminals to staff security checkpoints at Philadelphia airport even after being fined $1 million last year for failing to check the backgrounds of its workers.

The FAA-OIG investigators found this week that screeners at Seattle and some other airports had prior criminal records that should have prevented them from getting those jobs. Federal officials would not identify the other airports.

At Dulles, seven of 20 screeners given an annual written exam failed it and were assigned to other duties, investigators said. A screener must receive 12 hours of training, pass a written test and be retested every year.

Investigators found seven undocumented workers at Dallas screening passengers. Last month, the inspector general began investigating whether undocumented workers were working illegally at Dulles checkpoints.

Under federal law, only U.S. citizens or holders of green cards or work permits can work at security checkpoints, where all passengers and carryon baggage are screened for weapons.